Stainless steel



United States Patent STAINLESS STEEL Bruce F. Richardson, Ottawa, Ontario, Canada, assignor 'to Quebec Metallurgical Industries Ltd., Ottawa, 011- tario, Canada, a company. of Canada No Drawing. Application October 5, 1954 a 1 Serial No. 460,515

3 Claims. (Cl. 75-126) This invention relates to stainless chromium steels and more particularly to stainless chromium steels suitable for use in producing cutlery and surgical knife blades.

The advantage of stainless steel knife blades as compared to those of plain carbon steel has long been recog-nized. The stainless steel commonly used for knife blades prior to the present invention has been a low carbon chromium steel consisting by weight of about 11.5 to 15 percent of chromium, from about 0.15 to 2.00 percent of manganese, from about 0.15 to 1.00 percent of silicon, not more than 0.15 percent of carbon and the balance iron except for impurities including not more than about 0.04 percent of each of sulfur and phosphorus. knifeblades, the steel ingot is heated to a temperature between about 1010 C. and 1149 C. and subjected to a rolling operation to form asheet having a. thickness of substantially that of the desired knife blades. After annealing, knife blade blanks are cut from the sheet and hardened by heating above the critical temperature of the steel and then quenching. The hardened blades were subjected to a conventional drawing treatment to relieve strains set up therein by the hardening treatment.

The stainless steel knife blades thus produced were too soft to maintain a sharp cutting edge becausethe carbon content was too low. Prior to the present invention it had not been economically practical to increase the carbon content of this stainless steel above 0.15 percent because the steel could not be softened sufficiently to be subjected to the above mentioned rolling operation without requiring excessive power to drive the rolling mill or causing the rolls themselves to have a short life or producing edge cracking or other rolling flaws. It had previously been possible to produce knife blades of plain carbon steel having a sufficiently high carbon content to maintain a sharp cutting edge. However, when suflicient chromium was introduced into the steel to render it stainless, it was necessary to reduce'the carbon content to not more than 0.15 percent by weight to avoid an excessive power requirement for driving the rolling mill or causing the rolls themselves to have a short life and to avoid producing edge cracking and other flaws during the rolling operation.

-The present invention is based in part upon the discovery that by introducing a small amount of cobalt and a relatively smaller amount of boron into a chromium steel, an ingot of a stainless chromium steel containing up to about 1.00 percent by weight of carbon can be produced which can be softened sufliciently by heating to a temperature between 1010 C. and 1149 C. to be rolled to a sheet having substantially the thickness of a desired knife blade without requiring excessive power to drive the rolling mill or causing the rolls themselves to In accordance with one method for producing.

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have a short life or producing edge cracks or other rolling flaws. The invention also is based in part upon the further discovery that the presence of the small amount of boron has a twofold effect upon the steel. Thus, the boron appears to soften the steel lor rolling purposes when the steel is heated to a temperature of between 1010" C. and 1149" C. and the steel may be heated above its critical temperature and quenched to obtain a hardness comparable to that of a steel of the same carbon content and otherwise of substantially the same composition except that the chromium, cobalt and boron are omitted.

The invention particularly contemplates a stainless chromium steel having a carbon content of between about 0.55 to'0.85 percent by weight from which knife blades can be fabricated having a Rockwell C hardness at least 10 to 20 points greater than the low carbon chromium stainless steel knife blades theretofore produced. However, the invention may be used advantageously in stainless chromium steels having a carbon content as low as about 0.20 percent by weight since such steels often are difficult to roll without producing edge cracks or other rolling flaws.

In accordance with one aspect, therefore, the invention contemplates a stainless steel suitable for fabricating knife blades consisting by weight of from about 0.20

to 1.00 percent of carbon, from about 11.5 to, 15 percent I of chromium, from about 0.15 to 2.00 percent of manganese, from about 0.15 to 1.00 percent of silicon, from about 1 to 6 percent of cobalt, from about 0.001 to 0.005 percent of boron and the balance iron except for impurities including not more than about 0.04 percent of each of sulfur and phosphorus. The manganese and silicon impart no special properties to the steel of the invention and are used for the same purposes these elements are usually used in steel making. Their presence is tolerated within the limits specified.

The presently preferred knife blade composition consists by weight of about 0.60 percent of carbon, about 13 to 14 percent of chromium, about 0.60 percent of manganese, about 0.30 percent of silicon, about 3 to 5 percent of cobalt, about 0.001 to 0.005 percent of boron and the balance iron except for impurities including not more than about 0.04 percent of each of sulfur and phosphorus.

In its broader aspect the invention contemplates the production of a stainless steel article which must possess great hardness in its finished condition and which requires considerable mechanical working to form the article from an ingot or casting of the steel and which owes its stainlessness principally to the presence of chromium and its hardness principally to the presence of carbon. A knife blade is illustrative of such an article. Thus, the invention contemplates an article of this type fabricated from a steel containing as its essential components by Weight from about 11.5 to 15 percent of chromium and from about 0.20 to 1.00 percent and preferably from about 0.55 to 0.85 percent of carbon and which is capable of being softened for mechanical working by heating to a temperature between 1010 C. and 1149 C.

due to the presence of from about 1 to 6 percent and' preferably from about 3.00 to 5.00 percent by weight 1 of cobalt and from about 0.001 to 0.005 percent by weight of boron and which is capable of being hardened to a hardness equal substantially to that of a steel of the same carbon content and otherwise of the same composition except that the chromium, cobalt and boron are omitted by heating to a temperature above the critical temperature of the steel and quenching.

The invention is illustrated further by the following specific examples.

Example 1 A heat of steel was produced and divided to form two parts and each part was poured to form an ingot about 2% inches thick and about 4 inches wide and about 14 inches long. Boron was added to steel No. 1 in an amount of 0.0015 percent by weight, the steel otherwise consisting by weight of 0.65 percent carbon, chromium 14.01 percent, cobalt 5.00 percent, manganese 0.49 percent, silicon 0.30 percent and less than 0.04 percent of each of sulfur and phosphorus. Steel No. 2 had substantially the same composition except that its carbon content was between 0.32 and 0.38 percent and it contained no cobalt or boron.

In order to compare the ease of rolling of these two steels, each ingot was heated to a temperature of 1082 C. and immediately passed between the rolls of the same electrically driven high speed rolling mill nine times to form a sheet about of an inch thick. Each pass through the rolling mill reduced the thickness one quar- Pass Steel Steel No. 1 No. 2

The above table shows that the two steels had substantially equivalent rolling properties.

T he above sheet from steel No. l was reheated to 1082 C. and rolled down to a thickness of W of an inch without edge cracking or any other rolling flaw. The sheet thus formed was annealed at 788 C. for 5 hours and knife blades were formed from it ranging in sizes from a 4 /2 inch paring knife to a ham slicer 13 inches long. These blades then were heat treated by heating to 1038 C. and quenching in water. The hardness of the blades was 62 Rockwell C. They were then subjected to a drawing operation by heating at 232 C. for one hour and the Rockwell hardness was between 57 and 58. These blades were tested for stainlessness and were found to have good resistance to a saturated aqueous solution of sodium chloride, lemon juice, orange juice and vinegar.

Example 2 This example illustrates the influence of cobalt and boron on chromium stainless steel having a relatively low carbon content, c. g. in the vicinity of 0.25 percent carbon.

A heat of steel was produced having a base composition substantially the same as that of Example 1. The heat was divided in three portions and each portion was cast to form an ingot. Ingot No. 1 contained 0.24 percent carbon but no cobalt or boron. Ingot No. 2 contained 5 percent cobalt, 0.0015 percent boron and 0.25 percent carbon. Ingot No. 3 contained 0.15 percent carbon and no cobalt or boron.

In order to compare the relative ease of rolling of these steels, each ingot was heated to a temperature of 1082 C. and immediately passed between the rolls of the same electrically driven high speed rolling mill eleven times. The current in hundreds of amperes required in the various passes is shown in the following table.

Reduc- Ingot Ingot Ingot Pass N0. tion in No. 1 N0. 2 No. 3

Inches A s 4 4 3% ,s 4 43s 3% s 4 4 3% ,s a 4 s 5 in 4 is a 1 5 4% is r; 5% s s on an n is 6 /6 6 5 /5 7 6 5V1 It will be noted that ingot No. 2, which contained cobalt and boron, rolled much easier than ingot No. 1 which had almost the same carbon content but no cobalt or boron. Also, ingot No. 2 rolled almost as easily as ingot No. 3 which contained only 0.15 percent carbon.

The amount of conditioning required by the ingots for re-rolling also was measured. Ingot No. 2 required only superficial grinding while ingots Nos. 1 and 3 required grinding to the extent of /8 inch in depth in places or a total of 4 inch considering both sides taken together.

I claim:

1. A stainless steel suitable for knife blades consisting by weight of from about 11.5 to 15 percent chromium, from about 0.15 to 2.00 percent of manganese, from about 0.15 to 1.00 percent silicon, from about 1 to 6 percent cobalt, from about 0.001 to 0.005 percent boron and the balance iron and carbon except for impurities including not more than about 0.04 percent of each of sulfur and phosphorus, said carbon being not greater than 1.00 percent but in an amount corresponding to that which is effective to increase substantially the hardness of a steel of substantially the same composition but without said contents of chromium, cobalt and boron when the steel is heated above its critical temperature and quenched, said contents of cobalt and boron rendering the steel sufficiently soft when heated to a temperature between 1010 C. and 1149 C. to permit an ingot thereof to be subjected to a rolling operation to reduce it to a sheet of substantially the thickness of knife blades with out requiring excessive power to drive the rolling mill or causing the rolls themselves to have a short life or produeing edge cracking, the steel being capable of being hardened to a hardness equal substantially to that of a steel of the same carbon content but without said con tents of chromium, cobalt and boron by heating above the critical temperature of the steel and quenching.

2. A stainless steel suitable for knife blades consisting by weight of from about 0.55 to 0.85 percent of carbon, from about 11.5 to 15 percent of chromium, from about 0.15 to 2.00 percent of manganese, from about 0.15 to 1.00 percent of silicon, from about 3 to 5 percent of cobalt, from about 0.001 to 0.005 percent of boronand the balance iron except for impurities including not more than 0.004 percent of each of sulfur and phosphorus, said contents of cobalt and boron rendering the steel sufiiciently soft when heated to a temperature between 1010 C. and 1149 C. to permit an ingot thereof to be subjected to a rolling operation to reduce it to a sheet of substantially the thickness of knife blades without requiring excessive power to drive the rolling mill or causing the rolls themselves to have a short life or producing edge cracking, the steel being capable of being hardened to a hardness equal substantially to that of a steel of the same carbon content but without said contents of chromium, cobalt and boron by heating above the critical temperature of the steel and quenching.

3. A stainless steel suitable for knife blades consistingby weight of about 0.60 percent of carbon, about 13 to 14 percent of chromium, about 0.60 percent of manganese, about 0.30 percent of silicon, about 3 to 5 peraaaaote cent of cobalt, about 0.001 to 0.005 percent of boron and the balance iron except for impurities including not more than about 0.04 percent of each of sulfur and phosphorus, said contents of cobalt and boron rendering the steel sufliciently soft when heated to a temperature between 1010" C. and 1149 C. to permit an ingot thereof to be subjected to a rolling operation to reduce it to a sheet of substantially the thickness of knife blades without requiring excessive power to drive the rolling mill or causing the rolls themselves to have a short life or producing edge cracking, the steel being capable of being hardened to a hardness equal substantially to that of a 6. steel of the same carbon content but without said contents of chromium, cobalt and boron by heating above the critical temperature of the steel and quenching.

References Cited in the file of this patent UNITED STATES PATENTS 1,385,740 Armstrong July 26, 1921 2,531,720 Baeyertz et a1. Nov. 28, 1950 2,750,283 Loveless June 12, 1956 FOREIGN PATENTS 160,792 Great Britain Aug. 25, 1921 

1. A STAINLESS STEEL SUITABLE FOR KNIFE BLADES CONSISTING BY WEIGHT OF FROM ABOUT 11.5 TO 15 PERCENT CHROMIUM, FROM ABOUT 0.15 TO 2.00 PERCENT OF MANGANESE, FROM ABOUT 0.15 TO 1.00 PERCENT SILICON, FROM ABOUT 1 TO 6 PERCENT COBALT, FROM ABOUT 0.001 TO 0.005 PERCENT BORON AND THE BALANCE IRON AND CARBON EXCEPT FOR IMPURITIES INCLUDING NOT MORE THAN ABOUT 0.04 PERCENT OF EACH OF SULFUR AND PHOSPHORUS, SAID CARBON BEING NOT GREATER THAN 1.00 PERCENT BUT IN AN AMOUNT CORRESPONDING TO THAT WHICH IS EFFECTIVE TO INCREASE SUBSTANTIALLY THE HARDNESS OF A STEEL OF SUBSTANTIALLY THE SAME COMPOSITION BUT WITHOUT SAID CONTENTS OF CHROMIUM, COBALT AND BORON WHEN THE STEEL IS HEATED ABOVE ITS CRITICAL TEMPERATURE AND QUENCHED, SAID CONTENTS OF COBALT AND BORON RENDERING THE STEEL SUFFICIENTLY SOFT WHEN HEATED TO A TEMPERATURE BETWEEN 1010*C. AND 1149*C. TO PERMIT AN INGOT THEREOF TO BE SUBJECTED TO A ROLLING OPERATION TO REDUCE IT TO A SHEET OF SUBSTANTIALLY THE THICKNESS OF KNIFE BLADES WITHOUT REQUIRING EXCESSIVE POWER TO DRIVE THE ROLLING MILL OR CAUSING THE ROLLS THEMSELVES TO HAVE A SHORT LIFE OR PRODUCING EDGE CRACKING, THE STEEL BEING CAPABLE OF BEING HARDENED TO A HARDNESS EQUAL SUBSTANTIALLY TO THAT OF A STEEL OF THE SAME CARBON CONTENT BUT WITHOUT SAID CONTENTS OF CHROMIUM, COBALT AND BORON BY HEATING ABOVE THE CRITICAL TEMPERATURE OF THE STEEL AND QUENCHING. 